Wastewater treatment facilities separate the wastewater components into coarse solids, scum grit and sludge. Sewage sludge is a mixture of suspended, colloidal and dissolved organic and inorganic matter which is separated from wastewater during treatment.
Wastewater is generally subjected to a primary treatment wherein the suspended solid content is removed by physical means such as screening and gravity sedimentation. Chemical precipitation is useful in removing lightweight suspended and colloidal solids.
The remaining liquid sewage is then subjected to a secondary treatment wherein microorganisms, primarily bacteria, are used to stabilize and denature waste components by degrading complex organics and/or killing pathogens. The mixture of microorganisms is usually referred to as xe2x80x9cbiomass.xe2x80x9d During the biological treatment of wastewater or sewage, the waste components function as nutrients for the microorganisms, enabling them to reproduce and multiply as they stabilize and denature the waste components.
Thus, the quantity of biomass in a waste treatment system increases during the stabilization and denaturing treatment. In order to avoid the buildup of an excessive amount of microorganisms which can xe2x80x9cchokexe2x80x9d the process, a portion of the microorganisms must be removed or xe2x80x9cwastedxe2x80x9d from the treatment system. The wasted microorganisms are referred to as xe2x80x9cbiosludge.xe2x80x9d A major cost component of all biologically based processes is the need to dispose of this biosludge in an environmentally acceptable fashion.
The general treatment or management of sludge involves stabilization of biodegradable organics, concentration and deterring, and ultimate disposal of the stabilized, dewatered residue.
Generated sludges are often dilute, on the order of about 1-2 percent solids by weight. In order to reduce the volumetric loading on other processes, the first step in sludge processing is often concentration, by such means as gravity thickening and flotation.
Organic sludges from primary treatment can usually be concentrated to about 5-8 weight percent solids. Sludges from secondary treatment can usually be gravity thickened to about 2 to 4 weight percent solids.
Dewatering is different from concentration in that concentration still leaves the sludge with the properties of a liquid. Dewatering uses mechanical operations such as centrifugation, vacuum and/or pressure filtration and sand beds to produce a product which is essentially a friable solid. When the water content of sludge is reduced by dewatering to about 65-80 percent, it forms a porous solid called sludge cake. There is no free water in the cake as the water is chemically combined with the solids or tightly adsorbed on the internal pores or held within the cells of microorganisms.
Biosludge is the organic biomass remaining after biological waste water treatment. Biosludge, usually has to be mechanically dewatered to reduce water content below approximately 96 weight %. A significant source of water in the biosludge is contained within the cells of biota present in the sludge, and is referred to as xe2x80x9cintracellular water.xe2x80x9d A dewatered biosludge filter cake with no free liquid can still have a water content in excess of 80% by weight due principally to the amount of intracellular water contained in the cells of the dewatered biosludge. xe2x80x9cDry solidsxe2x80x9d is the water-free residue left after a sample is dried in nitrogen at 105xc2x0 C. until no further weight loss is observed. The term xe2x80x9cfree liquidxe2x80x9d is liquid that is not physically adsorbed or encumbered or chemically combined, and can be released through conventional filtration processes.
Currently available technologies for reducing the water content of the biosludge and utilizing the biosludge are costly and require excessive amounts of energy. In particular, they have all been found to be uneconomical and impractical means for converting biosludge and sewage sludge into a viable feed for a partial oxidation gasification reaction.
The high intracellular water content contained within the cell walls of the bacterial cells of a dewatered biosludge is reduced by removing the intracellular water in a denaturing operation. This operation comprises heating the biosludge at a temperature sufficient to weaken the bacterial cell walls. The weakened cell walls are then exposed to a reduced pressure sufficient to form vapor within the cell and to thus rupture the weakened cell walls and thereby release the intracellular water as free water or in the form of a hot aqueous vapor. The water-reduced concentrated biosludge can then serve as a fuel source in a partial oxidation reaction for the production of synthesis gas.